mpdparser: make durations unsigned where appropriate

mpdparser: make durations unsigned where appropriate

The standard does not seem to make any particular explicit not
implicit reference to the signedness of durations, and the code
does not rely on such, nor on the negativity of the -1 value
that's used as a placeholder when a duration property is not
present in the XML.

https://bugzilla.gnome.org/show_bug.cgi?id=750847